In present day television and video practice, the standard aspect ratio is 4:3, in accordance with EIA standard RS-170 and NTSC color television broadcast standards. However in the motion picture field, many movies have been made in a widescreen mode, typically with a 16:9 aspect ratio. Reproducing widescreen-formatted material on conventional video displays that are designed for the normal 4:3 aspect ratio has be problematic and has led to a number of adaptive compromises, both in film-to-video transfer and in reproduction in video display systems, that often result in loss of picture information, sacrifice of resolution capability and/or display at a grossly wrong aspect ratio, e.g. objectionably compressed horizontally. Thus, to date it remains as an unsolved problem to distribute and reproduce widescreen-formatted visual material satisfactorily on conventional video displays.
With recent developments in large screen video projection there has been a strong emphasis on quality refinements: one of these is the need for a video projection system that can operate optimally in displaying an image in at least a second aspect ratio mode in addition to the standard mode for which it is designed. It is highly desirable that such capability be obtained from a single projector, and for economic purposes it is highly desirable to be able to utilize an existing conventional projector designed for the standard 4:3 aspect ratio, and to enable it to project widescreen images (e.g. 16:9 aspect ratio) when required, without any degradation.
A video projector utilizes some form of image source screen such as a light-emitting phosphor screen of a CRT or an LCD light valve in the path of a beam projected from a light source, from which a source image on the source screen is projected optically onto a viewing screen. The source screen is dimensioned to match the aspect ratio of the source image, as dictated by the format of the video source material, typically the standard 4:3 aspect ratio. The outline of the projected image, being simply magnified optically by the diverging beam, is also displayed at the standard 4:3 aspect ratio.
Normally, registration of the source image on the source screen is controlled by adjusting the horizontal and vertical drive of a scanned raster applied to the source screen (e.g. by electron beam deflection in a CRT). The projector is designed to operate in the normal aspect mode with close outline registration between the raster, the source image and the source screen itself. However, in the usual manner of handling widescreen movies in video media and reproduction, the source image outline is allowed to become misregistered relative to the raster and the source screen, in effect underscanned vertically such that substantial regions of the source screen above and below the image remain unactivated. These blank regions represent loss of resolution capability and cause risk of burnin degradation, and yet any readjustment of the raster scan vertically and/or horizontally to address these problems in projection systems of known art will result in loss of picture information and/or introduce aspect ratio error in the projected image.
FIG. 1A shows the image of a simplified geometric test pattern 12A of standard 4:3 aspect ratio as displayed on a screen 10, e.g. a directly-viewed CRT screen or a projector source screen, of a conventional video display system. In accordance with best practice of known video art the CRT (or source screen) is operated in a manner to locate the produced image optimally registered on the active electro-optical region of screen 10 so as to fully utilize the electro-optical region and to display substantially all of the received picture information.
FIG. 1B illustrates a commonly used mode of adapting a conventional video projection system to display widescreen-formatted source material in a widescreen aspect ratio without loss of received picture information. For this example it is assumed that the original source material, shown here as a special widescreen test pattern, was transferred from film media with a 16:9 aspect ratio videotape media with the standard 4:3 aspect ratio by leaving blank regions 14 above and below the area occupied by the image 12B. Even though these regions 14 of the CRT or projector source screen 10 are raster-scanned in the display device, they are rendered inactive by the video signal being held at a black level.
The widescreen test pattern image 12B is displayed at the correct original aspect ratio 16:9 as evidenced by the correctly round circles, and there is no loss of picture information as evidenced by the completeness of the small circles displayed at each side. However this system has three disadvantages:
(1) There is a loss of vertical resolution capability, 25% in this instance, due to the wasted screen area in regions 16 and the increased pixel density in the central region.
(2) The screen 10 is placed at risk of screen burnin arising from the blanked operation of regions 16. Practically all known electro-optical materials such as CRT phosphors and LCD crystals suffer a gradual loss of efficiency due to electro-optical material fatigue over periods of operation. In the normal mode of FIG. 1A this deterioration tends to be distributed evenly throughout the panel area and thus may remain imperceptible and acceptable since generally it can be compensated for by a simple occasional adjustment of the projector thus extending the source screen's useful life. However prolonged operation as in FIG. 1B tends to "burn in" a permanent difference between the active region of screen 10 and the unused regions 14 that becomes unacceptable when the projector is subsequently operated in the standard mode of FIG. 1A. Burnin in is manifested as the appearance of unwanted borderlines and shadings that cannot be corrected by adjusting the projector, signifying the end of the screen's useful life. Projection systems, where small source screens are operated at extremely high intensity, are particularly susceptible to such burnin deterioration.
(3) In transitioning from standard mode to a widescreen mode, it is subjectively very desireable for the projected image area to actually become larger (i.e. wider); to the contrary, in FIG. 1B the projected image area becomes smaller than in the standard mode (FIG. 1A).
FIG. 1C depicts a mode of widescreen adaptation wherein the outline of screen 10 is in effect overscanned horizontally and underscanned vertically by the source image 12C, thus there is a loss both in vertical resolution capability due to blank regions 16 and in picture information: about 12.5% in each.
FIG. 1D depicts a mode of widescreen adaptation that allows screen 10 to remain fully scanned and activated: the image 12D fills screen 10 vertically, thus, with the correct aspect ratio preserved, about 25% of the original picture information is lost at the sides, as can be seen as loss of about half of the edge circles in the test pattern image 12D.
FIG. 1E illustrates a possible mode of operation wherein the aspect ratio of the widescreen-formatted image 12E has been intentionally offset in order to register image 12E optimally on the screen so that there is no loss of picture information or risk of burnin due to unused regions. The resultant aspect offset error compresses the image horizontally by 25%, distorting the circles to the elliptical shape seen in the test pattern image 12E.
Referring to the modes depicted in FIGS. 1A-E as modes A-E respectively, it is noted that any of modes B-E can be applied in a film-to-video transfer process: mode B is practiced frequently, mode C less frequently, and mode D rarely, while mode E is generally considered unacceptable and thus avoided in the practice of known video art because the large amount of aspect. It ratio error that is apparent in the resultant viewed image is further noted that only modes B and E preserve the full picture information: if modes C or D are applied in the original video production process, the lost picture information can never be recovered in reproduction. Operation in modes B or C, especially over prolonged periods of time, cause risk of burnin due to the blank top and bottom regions of the screen. Thus only mode E preserves full picture information and avoids source screen burnin, however it delivers incorrect aspect ratio, and furthermore there is at present no available widescreen video media formatted in this offset aspect ratio mode.
In general, absent the benefit of the present invention, whenever it is attempted to operate a source screen of a given aspect ratio from source material having a different aspect ratio, video projection systems of known art are subject to one or more of the following degradations: loss of resolution, loss of picture information at the sides, aspect ratio distortion (horizontal compression), and reduced life expectancy of the source screen due to misregistration burn-in.
In theory the aspect ratio of the projected image could be altered optically utilizing specially-shaped transparent lenses; however, these present numerous difficulties and disadvantages such as high cost, optical aberrations, the inability to change the displayed aspect ratio without physically interchanging lenses, and the inability to accomplish continuous variation of the aspect ratio.